Mode-locking is a technique that induces a fixed phase relationships among the modes of the laser's resonant cavity. As a result of mode-locking, the lasing modes interfere and a laser is caused to produce light output in a form of laser pulses of extremely short duration, for example on the order of picoseconds (10−12 s) or femtoseconds (10−15 s). Because of high peak power that is achieved due to the extremely short pulse duration, a mode-locked laser can be used for a variety of applications, including materials processing, spectroscopy, sensing, medicine, and light generation through optical nonlinearity.
A mode-locked fiber laser is a specific type of mode-locked lasers that include an optical fiber, made of appropriately doped glass, as a gain medium. Typically, for this purpose, a doped silica optical fiber is used having a length of several meters because of the relatively low gain per unit length the doped silica material. Such length of the optical fiber used as gain medium imposes a limitation on the repetition rate of a pulse train output of the mode-locked fiber laser of approximately 100 MHz.